A known diebond strip is shown in FIGS. 1 and 2. The strip 5 is typically an elongated rectangular shaped thin piece of graphite with a centrally located hole 20 and two further holes 22, 24 located at opposite edges of the strip. The diebond strip is typically 1 mm thick approximately 15 mm wide and 54 mm long. Typically a component to be attached to a substrate is placed over central hole 20 and held in place with vacuum pressure. The component is placed on the substrate using various known designs of ceramic tooling attached around the strip. The strip is heated by connecting a low voltage, high current supply to holes 22 and 24. These holes also function as a means of retaining the strip in place during the diebond process.
Currently, semiconductor laser die are mounted to a substrate by heating up an assembly in order to re-flow a preform of solder and allow the semiconductor laser die to be attached to the substrate. This process generates heat by passing a low voltage, high current supply along the die bond strip. With reference to FIG. 3, the semiconductor laser die 1, also know as a chip, in attached to a submount 2. The entire assembly 10 is heated via graphite strip 5 to a point where the solder preform 4 between the submount and the substrate reflows and attaches the chip-on-submount 2 to the substrate 3. However, a technical problem exits with this know process in that the temperature must not be sufficient to compromise the joint between the chip and its submount. Thus, accurate and consistent temperature control is required.
Current methods of attaching the chip-on-submount to the substrate do not use any active control on the graphite diebond strip. Instead, the system uses thermocouples 31, 32 as seen in FIG. 4, and periodically checks to ensure the temperature remains within set parameters.
Another technical problem associated with the current system is maintaining the contact between the thermocouple and the diebond strip, as the thermocouple cannot be attached reliably to the diebond strip. The position and contact of the thermocouple are maintained by using springs 33, 34. As a result, the control is based on the perceived temperature at the junction of the strip and thermocouple. Any variation in this junction can result in a variation of as much as 50° C., without any apparent change in the system.
Furthermore, the current system incorporates a parallel thermocouple system. Thus, if both thermocouples showed similar results, the system is considered to be “in balance” and ready for use. Any imbalance between the two thermocouples required the system to be re-balanced.
Yet another technical problem associated with the current diebond strip is that, due to the nature of graphite, it is difficult to press a thermocouple against the strip without the strip being flexed in some way. This reduces the effectiveness of the vacuum for the retention of the substrate on the strip, and compromises the transfer of heat energy from the strip to the substrate. Pressing the thermocouple against the strip also reduces efficiency of the thermocouple.
In addition, from the first time that the graphite strip is used, the strip begins to deteriorate. This is partially due to the processing of parts on the strip and causes an increase in the resistance of the strip. As the strip's effectiveness is reduced, and the process time gradually increases. This can be slightly compensated for by increasing the power to the strip. However, the current diebond strip must still be replaced approximately every 3 months.